Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor And The Related Insurance Loss Reserve On Company Insured Investments) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Schedule of Investments [Line Items]
Total, Fair Value	$ 1,558		$ 2,387
Total, Unrealized Losses	(226)		(478)
Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	478
Total, Unrealized Losses	(151)
Loss and loss adjustment expense reserves	18	[1]
Wrapped Securities [Member] | All Other [Member]
Schedule of Investments [Line Items]
Total, Fair Value	91
Total, Unrealized Losses	(11)
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	288
Total, Unrealized Losses	(119)
Asset-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	178
Total, Unrealized Losses	(77)
Loss and loss adjustment expense reserves	18	[1]
Asset-Backed Securities [Member] | Wrapped Securities [Member] | MBIA [Member]
Schedule of Investments [Line Items]
Total, Fair Value	147	[2]
Total, Unrealized Losses	(68)	[2]
Loss and loss adjustment expense reserves	18	[1],[2]
Asset-Backed Securities [Member] | Wrapped Securities [Member] | All Other [Member]
Schedule of Investments [Line Items]
Total, Fair Value	31
Total, Unrealized Losses	(9)
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	284
Total, Unrealized Losses	(63)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	200
Total, Unrealized Losses	(51)
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | MBIA [Member]
Schedule of Investments [Line Items]
Total, Fair Value	11	[2]
Total, Unrealized Losses	(1)	[2]
Mortgage-Backed Securities [Member] | Wrapped Securities [Member] | All Other [Member]
Schedule of Investments [Line Items]
Total, Fair Value	189
Total, Unrealized Losses	(50)
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Total, Fair Value	305
Total, Unrealized Losses	(31)
Corporate Obligations [Member] | Wrapped Securities [Member]
Schedule of Investments [Line Items]
Total, Fair Value	9
Total, Unrealized Losses	(12)
Corporate Obligations [Member] | Wrapped Securities [Member] | All Other [Member]
Schedule of Investments [Line Items]
Total, Fair Value	9
Total, Unrealized Losses	(12)
All Other [Member] | Wrapped Securities [Member] | MBIA [Member]
Schedule of Investments [Line Items]
Total, Fair Value	77	[2]
Total, Unrealized Losses	(11)	[2]
All Other [Member] | Wrapped Securities [Member] | All Other [Member]
Schedule of Investments [Line Items]
Total, Fair Value	$ 14

[1] Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.

[2] Includes investments insured by MBIA Corp. and National.